Peter's paradox 

The Peter paradox was coined when trying to explain how a person could simultaneously be incredible nice but also a massive dick. It is believed the occurrence of the Peter paradox is caused by a form of cognitive dissonance, brought on my long term emotional turmoil and misuse of alcohol. The term was named after the subject of the initial discussion, Peter.

"Jane was so supportive when me and Tom broke up, but now she wont stop making fun of me for going out with the jerk."
"Jane can be a real bitch some times, I just don't know why she does it."
"It's Peter's paradox Sarah, we'll just never know."

Peter's paradox by Kim41 August 18, 2013